a) Field of the Invention
The present invention relates to a zoom lens system which is suited for use with silver salt cameras and video cameras, and has a high vari-focal ratio.
b) Description of the Prior Art
It has conventionally been requested for photographic lens systems which are to be used with silver salt cameras, video cameras and so on to have high vari-focal ratios, compact sizes and high optical performance. As conventional examples of photographic lens systems which are configured so as to meet these requirements, there are known the zoom lens systems disclosed by Japanese Patents kokai Publication No. Sho 63-34,505, Kokai Publication No. Sho 60-11, 812 and Kokai Publication No. Sho 57-53,718. For obtaining a zoom lens system which can meet the above-mentioned requirements like the zoom lens system disclosed by these patents, it is conceivable to compose a zoom lens system, in order from the object side: of a first lens unit G.sub.1 which is to be kept stationary for zooming the lens system and has a positive refractive power; a second lens unit G.sub.2 which is referred to as a variator movable for zooming and having a vari-focal function, and has a negative refractive power; a third lens unit G.sub.3 which is referred to as a compensator movable for zooming and having a function to maintain an image surface at a constant location, and has a negative refractive power; and a fourth lens unit G.sub.4 which is to be kept stationary for zooming and has a positive refractive power. As a measure for configuring such a zoom lens system so as to have a short total length, it is known to strengthen the refractive power of the variator so as to shorten a distance for which this lens unit is to be moved for zooming the lens system. When the refractive power of the variator is strengthened, however, this lens unit produces aberrations in larger amounts which can hardly be corrected by itself.
For reducing variations of aberrations to be caused by zooming, it is generally desirable to correct aberrations in each of lens units which are used for composing a zoom lens system. From this viewpoint only, a large number of lens elements must be disposed in the variator for configuring a zoom lens system of the type illustrated in FIG. 1 so as to be compact and have high optical performance by strengthening the refractive power of the variator. However, such disposition of a large number of lens element in this lens unit is undesirable for configuring the zoom lens system compact. Though it is conceivable to use an aspherical surface or aspherical surfaces in the variator for favorably correcting aberrations without increasing a number of lens elements to be disposed in this lens unit, chromatic aberration cannot be corrected even when an aspherical surface or aspherical surfaces are used in the variator.